The overall goal of the proposed project is to investigate the microbial role in the biogeochemical cycling of a) chromium and b) arsenic in the environment. a) We intend to examine the detoxification of Cr(VI) to Cr(III) coupled to the concomitant degradation of co-contaminant chemicals, such as toluene, phenol or p-cresol, likely to be found in mixed wastes in hazardous waste sites. We hypothesize that the microbial degradation of the aromatic organics can be coupled to Cr reduction, that this process is potentiated by biotic and abiotic found in the environment, and that microbial activity can affect the availability and transport of these organic and inorganic contaminants in and through the environment. B) In addition, we intend to examine the diversity of microbial communities involved in and the extent of microbial arsenic transformations. We hypothesize that microbial activity enhances the rate of arsenic reduction, oxidation and methylation under environmental condition. We will select, characterize, and develop molecular biomarkers for environmental strains of microorganisms (both aerobic and anaerobic) which can metabolize toluene, phenol and p-cresol coupled to chromate (Cr(VI)) reduction; in addition, we will screen our large strain collection which degrade these aromatic contaminants for Cr(VI) reduction. We will compare the rate, extent and stoichiometry of Cr reduction in microcosms and by the pure cultures, and determine the environmental factors affecting its reduction. We will determine if microbial oxidation of Cr(III) is relevant to the transport and cycling of Cr in the environment. We will examine community structure and changes, and flux of reactants and products in laboratory column microcosms with Cr(VI) reducing microbial communities which metabolize organize co-contaminants. In addition, we will seek a wide microbial diversity of arsenic reduction in environmental samples and to compare the rate and extent of arsenic reduction in microcosms and by novel isolated pure cultures. We will also determine if microbial oxidation of As(III) is relevant to the transport and cycling of As in the environment and we will determine the role of sulfate reducers in arsenic methylation.